1125
Ind. Eng. Chem. Res. 1990,29, 1125-1129
be noted that SANS yields microstructural information, statistically representative of macroscopic sample volumes, in contrast to the detailed "local" information provided by for example electron microscopy. Acknowledgment We thank the Institut Laue Langevin, Grenoble, France, for use of experimental facilities and technical support. Two of us (D. R. Acharya and R. Hughes) also thank the SERC for financial assistance and Crosfields Catalysts for provision of catalyst samples. We acknowledge valuable discussions with Dr. A. F. Wright of the Institut Laue Langevin, Dr. A. T. Chadwick of Harwell Laboratory, and Dr. E. Evans of Kemeric. Part of the work described in this paper was undertaken as part of the underlying research program of the UKAEA. Nomenclature A = area of the scattering features, m2 d C /dQ = macroscopic differential small-anglescattering cross section, m-l sterad-' f = volume fraction of the scattering features, dimensionless H = mean capillary half-length, nm Q = scattering wavevector, nm-' r = radius of gyration for capillaries, nm R = radius of scattering features, nm R, = radius of gyration in the Porod model, nm R, = radius of gyration for globules, nm So= area of the interface per unit volume of the sample, m-l Greek Symbols
= neutron scattering length density of the inhomogeneities, m-2 p b = neutron scattering length density of the catalyst matrix, m-2 X = wavelength of the neutron beam, nm Registry No. Coke, 7440-44-0; xylene, 1330-20-7. pb
Literature Cited Acharya, D. R.; Allen, A. J.; Hughes, R. Characterization of Coke Deposita on Catalyst Using Small Angle Neutron Scattering. Intemal Report MPD/NBS/350; UKAEA Hanvell, England, Sept. 1988. Adams, C. R.; Voge, H. H. Aging of Silica-Alumina Cracking Catalysts. 11. Electron Microscope Studies. J. Phys. Chem. 1957,61, 722-727.
Allen, A. J.; Oberthur, R. C.; Pearson, D.; Schofield, P.; Wilding, R. C. Development of the Fine Porosity and Gel Structure of Hydrating Cement Systems. Philos. Mag. B. 1987,56, 263-288. Baston, A. H.; Potton, J. A,; Twigg, M. V.; Wright, C. J. Determination of Particle-Size Distributions of Heterogeneous Catalysts on High-Electron-Density Supports by Neutron Small-Angle Scattering: Dispersed Nickel Oxide on a-Alumina. J.Catal. 1981, 71, 426-429. Best, D. A.; Wojciechowski, B. W. The Catalytic Cracking of Cumene-The Kinetics of the Dealkylation Reaction. J. Catal. 1977, 47, 343-357. Blank, H.; Maier, B. Guide t o Neutron Research Facilities; ILL: Grenoble, France, 1988. Brumberger, H.; Goodisman, J. Voronoi Cells: An Interesting and Potentially Useful Cell Model for Interpreting the Small Angle Scattering of Catalysts. J. Appl. Crystallogr. 1983, 16, 83-88. Delgass, W. N.; Wolf, E. E. Catalytic Surfaces and Catalyst Characterization Methods. In Chemical Reaction and Reactor Engineering; Carberry, J. J., Varma, A., Eds.; Marcel Dekker: New York, 1986. Espinat, D.; Moraweck, B.; Larue, J. F.; Renouprez, A. J. Determination of Particle Distribution in Supported Metal Catalysts by Small-Angle Scattering. J.Appl. Crystallogr. 1984, 17, 269-272. Froment, G . F.; Bischoff, K. B. Non-Steady State Behavior of Fixed Bed Catalytic Reactors due to Catalyst Fouling. Chem. Eng. Sci. 1961, 16, 189-201. Guinier, A.; Fournet, G. Small Angle Scattering of X-RQYS;John Wiley: New York, 1955. Gunn, E. L. The Quality and Structure of Catalyst by X-Ray Low Angle Scattering. J. Phys. Chem. 1958, 62, 928-934. Haldeman, R. G.; Botty, M. C. On the Nature of the Carbon Deposit of Cracking Catalyst. J. Phys. Chem. 1959, 63, 489-496. Koester, L.; Yelon, B. A Compilation of Neutron Scattering Data; Netherlands Energy Research Foundation: Petten, The Netherlands, 1982. Massoth, F. E. Oxidation of Coked Silica-Alumina Catalyst. Ind. Eng. Chem. Process Des. Deu. 1967,6, 200-207. Somorjai, G. A,; Powell, R. E.; Montgomery, P. W.; Jura, G. Small Angle X-Ray Study of Metallized Catalysts. In Small Angle X-Ray Scattering; Brumberger, H., Ed.; Gordon and Breach New York, 1967. Takahasi, T.; Kodama, T.; Watanabe, K. Deactivation of Silica Alumina Catalyst in the p-xylene Isomerization Reaction. J.Jpn. Pet. Inst. 1978,21, 2-6. Vonk, C. G. On Two Methods for Determination of Particle Size Distribution Functions by Means of Small Angle X-Ray Scattering. J. Appl. Crystallogr. 1976, 9, 433-440. Wolf, E. E.; Alfani, F. Catalyst Deactivation by Coking. C a t d Reu.-Sci. Eng. 1982,24, 329-371. Receiued for review August 30, 1989 Accepted February 5, 1990
Ammonia Oxidation Catalysts with Enhanced Activity Robert J. F a r r a u t o * and Hyo
C.Lee
Engelhard Corporation, Research & Development, Menlo Park, Edison, New Jersey 08818
Platinum alloy gauzes, used as catalysts for nitric acid production, experience a significant increase in surface area during their first days on-stream. This well-known phenomenon, called sprouting, results in a 20-fold increase in surface area but often requires several days to reach a maximum. Since high surface area is vital for the ammonia oxidation activity, this represents a significant loss in acid production during a typical 45-90-day cycle in a high-pressure (Le., 8-10-atm) plant. This paper describes the laboratory and field data generated from a new Engelhard patented Hylite manufacturing procedure which produces a catalyst with enhanced activity, shortening the time to reach the maximum production of nitric acid. The manufacturing methods, enhancement in activity, and improvements in steady-state operation will be discussed. Introduction Platinum alloy gauzes, in contrast to most metal-supported or metal oxide supported catalysts, increase in
* T o whom correspondence
should be addressed.
0888-5885/90/2629-1125$02.50/0
catalytic surface area during ammonia oxidation in the production of nitric acid (Anderson, 1988). This wellknown sprouting phenomenon has been studied for many years (Sperner and Hohmann, 1976; McCabe et al., 1986; Schmidt and Luss, 1971). Although the precise mechanism 0 1990 American Chemical Society
1126 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
believed operative is still subject to dispute, unquestionably morphological changes occur on the surface of the smooth platinum alloy wires, resulting in considerable grain growth and increased surface area. It is clear that these changes are accelerated due to the interaction of reactants and/or products with "platinum oxides" produced during the oxidation. The highly exothermic nitric acid production process is carried out between 810 and 960 "C at pressures varying from 1 to 10 atm with 10-12% ammonia and the balance air. Accompanying the enhanced increase in surface area, there is a considerable loss of platinum in the form of volatile platinum oxides, which is now commercially recovered by the installation of getter gauzes (Holtzmann, 1969; Hatfield et al., 1983) immediately below the oxidation gauze catalyst. During the early stages of the process, the activity gradually increases in parallel with the growth in catalytic surface area of the gauzes. Since the reaction is controlled a t steady state by bulk mass transfer, any increase in geometric surface area favors enhanced activity. Thus, performance is not maximized until the gauze is fully sprouted and the catalytic area has reached a maximum. In a typical high-pressure plant (Le., 8-10 atm), this requires 3-5 days, representing a significant loss in production during the typical 45-90-day life cycle of the catalyst. This paper describes a surface treatment of the catalyst wire gauze during its manufacture, which enhances activity during the 3-5 days between start-up and steady-state performance, resulting in higher nitric acid production. Background Conventional Catalyst. Platinum alloyed with 10% rhodium (Hanforth and Tilley, 1934) or 5 % rhodium and 5% palladium (Powell, 1945) are used as the oxidation catalysts throughout the world. Typically, the catalyst consists of smooth wires 0.076 mm in diameter woven to give 1024 mesh/cm2. The purified metals are drawn into wires on conventional wire drawing machines and are frequently treated to remove oils and other contaminants from the surface. The woven gauze is produced on a conventional loom; however, the final pattern can be varied for specific applications. Process Description Plants vary in size and pressure based on capital and operating costs. In the US., nitric acid is typically produced in high-pressure plants (i.e., 8-10 atm) utilizing three component gauzes (i.e., platinum, rhodium, and palladium) approximately 1-1.5 m diameter (or flat to flat for a hexagonal configuration). Fifteen to 30 sheets of gauze are stacked onto a support grid usually made of Inconel or other high-temperature, corrosion-resistant alloy. Frequently, palladium-rich alloy gauzes of varying geometries are located immediately below the oxidation pack to recover the platinum volatilized during the oxidation process. This arrangement has been described elsewhere (Lee and Farrauto, 1989). Start-up is achieved by directing a hydrogen torch to a spot on the top sheet of the gauze pack and slowly introducing the ammonia-air mixture. Catalytic reaction is initiated at the heated spot, and the reaction wave slowly propagates radially across the top sheets of the gauze. During this time, which can be a few hours, a considerable amount of ammonia passes unreacted through the unheated portion of the gauze pack, resulting in a significant economic penalty. Worse yet, the bypass ammonia can catalytically react with NO on the surface of the palladium recovery gauze, leading to the formation of N2 and creating
a hot spot, the intensity of which can cause melting of the getter (Lee and Farrauto, 1989). During the next 3-5 days, the conversion gradually increases as the surface area is further developed. The NO produced by the reaction 4NH3 + 5 0 2 4N0 + 6H20 is cooled and noncatalytically converted to NO2 2N0 + 02 2NO2 During water absorption, NO2or its dimerized form, N204, reacts with H20,forming HNO, and additional NO, which is later reoxidized and ultimately converted to acid (Nitrogen, 1985). 3N02 + H 2 0 2HN03 + NO 3N204 2H2O ----* 4HNO3 + 2N0
-
+
-
-
Technical Approach There are definite economic and technical incentives to enhance light-off and shorten the time necessary for a fresh gauze to reach its fully developed morphological state where its surface area reaches a maximum. By doing so, full production of nitric acid is reached more quickly due to the maximum mass-transfer-limiting conversion of ammonia to nitric oxide. Furthermore, if light-off times could be reduced, little ammonia would bypass the oxidation gauze, react with NO, and potentially damage the recovery palladium getter gauze. Finally, by minimizing the time the hydrogen torch is directed to the oxidation gauze, the risk of a thermal deformation of the platinum alloy catalyst will be reduced, a problem occasionally observed in commercial installations. Instinctively, catalytic scientists try to increase the active site concentration by dispersing the catalytic components onto supports, increasing the accessibility to the reactants. Consequently, it was reasoned that, if platinum atoms could be dispersed onto the surface of the smooth wire, high concentrations of active sites could be produced during catalyst manufacturing. This would initially create a high surface area, simulating sprouting and minimizing the economic and technical penalties normally occurring in the early stages of nitric acid production. A variety of methods were therefore investigated to deposit a high surface area platinum coating uniformly onto the smooth surfaces of the gauze (Farrauto et al., 1989). Experimental Section 1. Catalyst. High surface area coatings were produced by depositing platinum salts onto the surface of a Pt alloy gauze from aqueous or nonaqueous solutions. Both binary (Pt/lO% Rh) and ternary (Pt/5% Rh/5% Pd) production alloys were used; however, all gauzes were 1024 mesh/cm2 with wires of 0.076-mm diameter. Prior to receiving the coating, the gauzes were cleaned in an ultrasonic bath, washed with deionized water, and dried in air. Chloroplatinic acid was dissolved in water or butanol (5% platinum solution by weight) and sprayed onto the gauze surface. The solvent was removed by oven drying at 120 "C, and the platinum salt was decomposed to metallic platinum by heating in air to 450 "C, leaving a gray, grainy platinum surface. Butanol as a solvent was found to produce more uniform high surface area platinum coatings than water or water with a surfactant, i.e., Igelpol CO-630 or Colloid 211. Electrostatic spraying also proved to be an effective deposition technique. The surface areas of the gauzes were measured by using an electrochemical technique described by Anderson (1988). This method permits accurate measurements of surface areas below those detectable by the standard BET
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1127 Table I. Pt-Coated Gauzes Prepared from Aqueous CPA" Solution
coating sample Pt salt method 1 CPA dipping
Pt loading, g/m2
surface area, cm2
0.47
63
2 3 4 5
CPA CPA CPA CPA
air spray air spray air spray electrostatic deposition
2.15 4.64 1.04 1.58
365 400 763 1228
6
CPA
electrostatic spray
1.21
495
7
CPA
electrostatic spray
5.34
514
a
remarks soaked overnight
2.5 A/ft2 for 30
min one coat on one side two coats on one side
Un- Coated Gauze
CPA = chloroplatinic acid.
procedures used for high surface area supported catalysts (Farrauto and Hobson, 1987). 2. Reactor and Test Procedures. Details of the reactor design were presented elsewhere (Heck et al., 1982). A hydrogen torch, used in commercial installations for ignition, is not present in the laboratory reactor. Laboratory ignition tests were conducted by loading five sheets of coated gauze into the reactor chamber. A thermocouple was embedded between the second and third layers of the gauze to determine the minimum temperature for light-off of the ammonia oxidation reaction. Air, a t a fixed preheated temperature, was passed through the gauze, and pulses of ammonia were injected into the air stream to give a concentration of 2.5%. If no exotherm was noted, the air preheat was increased and again ammonia was injected. This procedure was repeated until a definite sustained exotherm was noted. All tests were conducted a t 100 psig and a flow rate of approximately 12 kg/(m2*s). After ignition, the ammonia concentration was immediately increased to 11% , and the preheat temperature decreased to 200 "C, thereby allowing the conversion to proceed toward steady state. Analysis was conducted by conventional titrations after gases were collected in glass sample bulbs (Heck et al., 1982). Results and Discussion 1. Coating Deposition Techniques. Three methods were examined to produce high surface area coatings from aqueous solutions of chloroplatinic acid: dipping, air spraying, and electrostatic deposition (Table I). Dipping was the least effective, producing low loadings of platinum (i.e., 0.47 g/m2) with low surface areas, 63 cm2/g. Air spraying gave higher loadings with higher surface areas but required multiple sprayings on each side of the gauze and produced coatings with poor uniformity. The most effective procedure, both from coating quality and efficiency, was electrostatic deposition. High loadings, 1.58 g/m2, and areas, 1228 cm2/g, could be obtained by simply spraying one side of the gauze and allowing the coating to migrate to the opposite side. Further improvements in coating quality and reproducibility could be achieved using butanol as a solvent for chloroplatinic acid as well as other chloride-free salts. A typical section of coated gauze wire is shown in Figure 1. 2. Ignition Temperature and Surface Area. Figure 2 plots the ignition temperature ("C) verses the metal surface area (cm2/g). Also shown in the Figure 2 is the ignition temperature of a gauze directly from production
,
Coated
Gauze
Figure 1. Comparison of gauze morphology (magnification, 700X). 360
Un-tmated Gauze
1W-C QndfifUM
300
It 200
J
Prerrure 6 0 0 kPa Ammonia Feed 2.6% Marr Flux 11.6 Kg/mZr Gauze Arrangement : 00% wion ~h 1024 merher/c 0.076 mm dla. 6 ~ayerr
*
j
m
Q
200
400 800 000 Pt 8urf.c. Aror ( c d / o )
1000
1200
Figure 2. Ignition temperature versus surface area.
with no pretreatment (firing or acid washing) to remove contaminants. The ignition temperature is 350 "C. The same gauze treated in boiling hydrochloric acid to remove surface contaminants had an ignition temperature 50 "C lower. Both had surface areas of about 13 cm2/g. Higher surface area coatings were obtained by increasing the loading of metal deposited on the gauze. In each case, a 5% platinum solution of chloroplatinic acid was dissolved in butanol and air sprayed onto the acid-treated gauze. Clearly the ignition temperature decreases from almost 300 "C to about 230 "C a t coating surface areas in excess of 500 cm2/g. 3. Yield and Surface Area. The NO, yield of a conventional three-component gauze is slowly increased with on-stream time as shown in Figure 3. Here the inlet
1128 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
ii
P
l
i
1
," Uncoated Oaun,
4oo"cl
II
, /
10-
j
A
iE
C08t.d( 566 o&p)
66
0
1
2
3
4
O n - S t r ~ r iTime (hr)
Figure 3. NO, yield profile during initial start-up (laboratory).
.-
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0
I
Amnonla Conwrrlon
Figure 4. Ammonia reaction rate profile.
temperature is fixed at 200 "C since the NH3concentration is 11%and the pressure is 100 psig. Initially, the gauze surface area is 13 cm2/g and gradually increases as the sprouting process occurs (Anderson, 1988). Simultaneously the growth in the surface area and the activity reach a maximum in about 4 h (open circles). It should be pointed out that these data were generated in our pilot plant and that due to its smaller size, uniform mixing, and feed presentation to the catalyst gauze, equilibration is considerably more rapid than in a commercial plant. Typically, maximum conversion to NO, is achieved in 4 h in the pilot plant reactor versus about 4 days in a commercial reactor. A gauze coated with platinum with a surface area approaching 2000 cm2/g is plotted as solid squares. Immediately after light-off, the yield is seen to climb rapidly toward a maximum. Within 30 min, the coated gauze is producing almost 98% NO, while the conventional gauze is producing less than 90%. After approximately 1h, the coated gauze has leveled at almost 99%, where it remains for the duration of the run. The conventional gauze reaches about a 93% yield in 2 h and 98% after 3 h. Also shown as a solid triangle is a coating of 385 cm2/g, which shows the earlier yield advantage over the conventional gauze, but it is much less than its higher surface area counterpart. After approximately 3 h, the advantage of the coated gauze disappears and both coated and uncoated gauzes have comparable surface areas of about 250 cm2/g. Thus, the high area coating is only effective during the start-up phase of the process since it sinters and then dissolves into the sprouting base gauze as the surface temperature approaches 860 "C. 4. Ignition Characteristics. When ammonia is mixed with preheated air and passed over a Pt/Rh/Pd gauze bed, ignition occurs a t the gauze surface, causing a sharp rise in its surface temperature. Once ignited, the gauze temperature becomes sufficiently high that ammonia conversion becomes controlled by the mass-transfer rate. Under mass-transfer-controlling conditions, a hot thermal boundary forms a t the gauze surface. The surface then approaches an adiabatic equilibrium temperature due to an insufficient convective heat-transfer rate relative to the highly exothermic heat release. Accordingly, ignition characteristics are primarily controlled by temperature, ammonia concentration, mass throughput rate, and catalyst surface morphologies. The kinetics of the ammonia oxidation reactions over polycrystalline Pt/Rh/Pd gauzes have been reported to
obey a Langmuir-Hinshelwood mechanism in which ammonia and dissociated oxygen adsorb noncompetitively (Pignet and Schmidt, 1975). Using these kinetic data, the reaction rate profiles for ammonia oxidation over a gauze pad are plotted as a function of the ammonia conversion in Figure 4. Ignition characteristics over the Pt/Rh/Pd gauze are primarily controlled by ammonia and nitrogen oxide interactions below 300 "C. Experimental data for ignition indicate that the coated gauze yields approximately 25 times higher kinetic activities based on the bulk gauze surface area. Using 5% ammonia feed, the mass-transfer correlation data (Satterfield and Cortz, 1970) exhibit a significant ammonia conversion for ignition over a coated gauze layer above 300 "C. Such an improvement in gauze ignition characteristics cannot only alleviate difficulties in commercial reactor start-ups but also contribute to rapid development of proper morphologies at the gauze surface during the early stage of a gauze cycle. 5. Stability. Extinction occurs when the hot thermal boundary generated by the exothermic catalytic reactions cannot be sustained. This is a common occurrence during start-up of a nitric acid plant. External heat is provided by a hydrogen torch directed toward the surface of the gauze. If the torch is prematurely removed, the surface reactions may not be of sufficient rate to sustain the hot boundary layer and the reaction is extinguished. The hydrogen torch manipulation must then again be repeated. Furthermore, low flow rates of NH3 during start-up result in poor mixing, and thus extinction can occur due to a localized low concentration of NH3. The pilot plant reactor, in which most of the data here were generated, is not equipped with a hydrogen torch, and thus, procedures were developed to simulate ignition during start-up. In the pilot plant reactor, the air is preheated prior to mixing with the NH3until ignition is observed. For a conventional uncoated gauze, this occurs between 300 and 350 "C (dotted line) as shown in Figure 5. Extinction is evident when the air preheated is reduced to slightly above 200 "C. Obviously, the surface reaction rates are insufficient to sustain the hot boundary layer below this temperature. In contrast, the coated gauze (solid line) ignites a t 250 "C but does not extinguish until 35 "C. The catalytic reactions have been so enhanced by the coating in order to sustain the ignition over a wide temperature range, ensuring stable, one-time start-ups. A rapid, one-time
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1129 700 -
a
400-
Unoortmd Qru 200-
2
100-
I;I
c
loo
0
so0
200
400
Air Pnhmrt Tamp
Figure 5. Comparison of gauze stability.
0
10
100
110
of the new gauze relative to conventional gauze in just the first 5 days on-stream. Another legs quantifiable advantage demonstrated in the trial for the coated gauze is its ease in light-off. Here one relies on operator input regarding the process start-up. In this and other trials, it is always reported that less operational time in the use of the hydrogen torch is required. A rapid, smooth light-off translates to less ammonia bypass during start-up and less permanent damage to the gauze as a result of lengthy requirements of hydrogen torch operation. The latter should result in longer cycle lives, a prediction currently under study in this and other trials. Registry No. HN03, 7697-37-2; Pt90/Rh10, 11125-17-0; Pt90/Rh5/Pd5, 77981-46-5; Pd, 7440-06-4; NHs, 7664-41-7. Literature Cited
200
210
H w r i Owitreem Figure 6. NO, yield profile during commercial trial.
start-up in a commercial reactor usually results in a satisfactory, stable production run. 6. Commercial Experiments. The laboratory results reported above have been successfully translated to a commercial installation. A nitric acid producer installed both conventional and new coated gauze in consecutive plant trials (Lang et al., 1988). The plant capacity is 318 tonslday of nitric acid and operates at a pressure of 9.5 atm. The gauzes were hexagonal, 1048 mm from flat to flat. Data were collected every hour for the first 8 days on-stream with conversion efficiencies calculated twice a day. Calculations for efficiency were then made once daily for the remainder of the run. The results of these plant runs are compared Figure 6. Clearly, the coated new gauze performs with higher efficiencies during the first 200 h (i.e., 10 days) of operation. The observed efficiencies translate to a 3.3% improvement
Anderson, D. R. Catalytic Etching of Platinum Alloy Gauzes. J. Catal. 1988,113,475-489. Farrauto, R. J.; Hobson, M. Catalyst Characterization. In Encyclopedia of Physical Science and Technology; Meyers, R. A., Ed.; Academic: New York, 1987; Vol. 2, pp 563-589. Farrauto, R. J.; Lee, H. C.; Hatfield, W. R. Low Temperature Lightoff Ammonia Oxidation. US. Patent 4,863,893, 1989. Hanforth, S. L.; Tilley, J. N. Catalysts for Oxidation of Ammonia to Oxides of Nitrogen. Ind. Eng. Chem. 1934,26, 1287-1292. Hatfield, W. R.; Heck, R. M.; Hsiung, T. Method for Recovering Platinum in a Nitric Acid Plant. US. Patent 4,412,859, 1983. Heck, R. M.; Bonacci, J.; Hsiung, T. A New Research Pilot Plant Unit for Ammonia Oxidation Process and Some Gauze Data Comparisons for Nitric Acid Processes. Ind. Eng. Chem. Process Des. Dev. 1982, 21 (l),73-79. Holtzmann, H. Platinum Recovery in Ammonia Oxidation Plants. Platinum Metals Rev. 1969, 13, (l),2-8. Lang, M. A,; Lee, H. C.; Morse, R. W. New Ammonia Oxidation Catalyst. Nitrogen 88, British Sulfur 12th International Conference, Geneva, Switzerland, March 27-29, 1988. Lee, H. C.; Farrauto, R. J. Catalyst Deactivation Due to Transient Behavior in Nitric Acid Production. Ind. Eng. Chem. Res. 1989, 28 (l),1-5. McCabe, A. R.; Smith, G. D. W.; Pratt, A. S. The Mechanism of Reconstruction of Rhodium-Platinum Catalyst Gauzes. Platinum Metals Rev. 1986,30 (2), 54-62. Oxidation and Absorption in Nitric Acid Processes: Costing the Pressure Balance. Nitrogen 1985, No. 154 (March-April), 28-36, Pignet, T.; Schmidt, L. D. Kinetics of NH3 Oxidation on Pt, Rh and Pd. J. Catal. 1975,40, 212-225. Powell, A. R. Improvements in the Oxidation of Ammonia to Oxides of Nitrogen. U.K. Patent 570,071, 1945. Satterfield, C. N.; Cortz, D. H. Mass Transfer Characteristics of Woven-wire Screen Catalysts. Znd. Eng. Chem. Fundam. 1970, 9 (4) 613-620. Schmidt, L. D.; Luss, D. Physical and Chemical Characterization of Platinum-Rhodium Gauze Catalysts. J. Catal. 1971,22,269-279. Spemer, F.; Hohmann, W. Rhodium-Platinum Gauzes for Ammonia Oxidation. Platinum Metals Reo. 1976,20 (l),12-22.
Received for review August 8, 1989 Revised manuscript received February 19, 1990 Accepted March 5, 1990
